1. Field of the Invention
The present invention relates to a processing process of a master for optical disk. And more particularly to a processing method of the master for optical disk, a processing device which enables the processing method, and the master for optical disk produced by the processing method and a substrate for optical disk shaped by using the master.
2. Description of the Related Art
As a recording format for increasing recording density of an optical disk, a land and groove format is known in the art. This recording format records information both on a spiral or a concentric circular groove track which is formed along grooves, and a spiral or concentric circular land track which is formed along lands neighboring the grooves. Moreover, a prepit group for recording pre-formatted information, such as address information and a synchronous pattern for controlling a revolution of the disk is formed on a header of each groove and land. In order to make the signal levels of the groove track and the land track equivalent, the width of the groove and the width of the land are formed nearly equal. On the other hand, the width of the prepit group is narrower than the width of the groove.
In order to process such physical format, it is known to expose photosensitive material on a glass substrate for master by using three laser beams:
a laser beam for processing the grooves;
a laser beam for processing the prepit groups on the same spiral or circumference as the groove track; and
a laser beam for processing the prepit groups on the same spiral or circumference as the land track.
However, there is a problem in that according to such related art, an optical system of a mastering device is enlarged and complicated. Moreover, it is difficult to prevent the interference of the laser beams. Further, since the laser beam for processing the prepit groups on the groove track and the laser beam for processing the prepit groups on the land track are different, a difference in the qualities of the laser beams is easily generated. Furthermore, an effort in uniforming the qualities of the prepit groups causes a difficulty in optimizing the qualities of the grooves.
On the other hand, it may be considered to process the grooves and prepit groups by one laser beam by using an optical deflector. However, since it is necessary to make the widths of the grooves and prepit groups different, optimizing the qualities of the grooves and prepit groups is difficult. Moreover, while the amount of deflection of the laser beam in the situations of processing the prepit groups on the groove track and processing the grooves is nearly 0, the amount of deflection of the laser beam in the situation of processing the prepit groups on the land track should be nearly the same as a track pitch. In this case, differences in the diffraction efficiencies, an optical aberration, etc. are raised by the deflection, which cause a problem that the prepit characteristics will become unbalanced.
The present invention solves the aforementioned problems by forming the prepit groups by one laser beam to a master for optical disk of the aforementioned physical format. Moreover, the present invention makes it possible to form a high quality and well balanced prepit groups on a groove track and a land track by laser processing. Further, the present invention conducts groove processing by another laser beam. Accordingly, it is possible to form the laser beams, which suit each of the desired characteristics.
Moreover, the present invention provides a mastering device having an optical deflector on an optical course of a laser optical system for processing the prepit groups. The shooting position, in the event of no optical deflection of the laser beam, is set to the different position from the shooting position of the laser beam for processing grooves by nearly a half of a track pitch in a radial direction. In other words, the laser beam for processing the prepit groups is positioned to shoot on the middle position between the groove track and the land track.
Further, the optical deflector deflects the laser beam for processing the prepit groups in the same amount both to the position of the groove track and to the position of the land track. Furthermore, a subtle difference in diffraction efficiencies caused by a difference in the deflection directions is corrected and optimized by controlling the laser power independently from each other. Moreover, the laser power is respectively controlled in accordance with the length of each of the prepits on the groove track and the land track.